Changes in intrauterine pressure after oxytocin administration in reproductively normal mares and in those with a delay in uterine clearance

Intrauterine pressure was measured in 4 reproductively normal mares and 4 m ares with delay in uterine clearance after administration of oxytocin to de termine if intrauterine pressure varied between dosage and group. Changes i n intrauterine pressure were measured during estrus, when a follicle was gr eater than or equal to 35 mm, using a Millar "Mikro-tip"(R) catheter that h ad 3 discrete pressure sensors/channels. Mares received 4 different treatme nts of 10, 5, 2.5 or 0 IU (vehicle) of oxytocin. The protocol for each trea tment consisted of a 10-min baseline recording, administration of treatment and measurement of changes in intrauterine pressure for 65 min. After admi nistration of the first two treatments, mares were rested for 2 h and the p rotocol repeated for the remaining 2 treatments. Changes in intrauterine pr essure were measured on a physiograph and stored in a computer. The results were analyzed by 4x4 Latin Square Design analysis of variance (ANOVA) usin g the GLM procedure of the Statistical Analysis System. The ANOVA detected a main effect of treatment (P <0.01) and mare (nested within group; P <0.01 ) but no effect of channels, group or treatment-by-group interaction. There was a dose-dependent increase in uterine activity in both normal mares and those with delayed uterine clearance. A dose of 10 TU of oxytocin induced a larger number of uterine contractions (5.67 +/- 0.06) for a longer time ( 24.09 +/- 1.18 min) than the 5 IU (4.16 +/- 0.06 contractions and 16.31 +/- 1.18; P <0.01 min) or 2.5 IU dose (4.08 +/- 0.06 contractions and 17.61 +/ - 1.18 min). The first intrauterine wave occurred most often near the tip o f the horn in 10 of 12 recordings in normal mares and in 8 of 12 recordings in mares with delayed uterine clearance. It was then propagated from the m iddle of the horn to the uterine body just cranial to the cervix. There was no pattern of propagation for subsequent intrauterine pressure waves. We c onclude that the difference in spontaneous clearance of the uterus between the 2 groups is not reflected in their response to exogenous oxytocin as de termined by changes in intrauterine pressure. (C) 1999 by Elsevier Science Inc.